Cultures of normal metazoan cells have a limited, finite proliferative ability, which may be related to the normal aging process in metazoa. The ciliate protozoan Paramecium tetraurelia shows a clonal life cycle functionally analogous to that of normal metazoan cells. Starting at fertilization, a clone goes through a series of phases ending in clonal senility and eventual death. Because they possess many advantages in terms of culturing, handling and potential for genetic analysis, the clonal aging process in paramecia can be considered to be a useful model system for the apparently similar processes seen in metazoan cells. Hypotheses of the cause of clonal senility in P. tetraurelia fall into 2 major groups: 1) the accumulation of damage in the genome in the macronucleus, and 2) the development of a cytoplasmic (non-nuclear) condition which will not support the function of even a "good" nucleus. From the literature, evidence can be found to support either hypothesis. Either hypothesis can be tested using paramecia and a microinjection protocol which transfers nuclei or cytoplasm from young to old cells or from old to young cells. By thus effectively separating the nucleus from the cytoplasm, the procedure tests independently their individual contributions to the aging process. Tests performed under the first year of support, and still underway, adddressed the contribution of the cytoplasm to the aging process. Within the limitations of the testing procedure, the cytoplasm was not found to give any significant contribution to the aging of paramecial clones. This conclusion is somewhat tentative, since tests are still underway, but seems reasonably certain. For the period of requested support in this continuation, the much more difficult tests using nuclear transplantations are proposed. If the macronucleus of paramecia is the primary site of aging, then transferring a young nucleus into the cyoplasm from an old clone should produce a hybrid clone which is young (i.e., will have a long life-span). Conversely, placing an old macronucleus into a young cytoplasm should produce an old hybrid. Appropriate controls will be done at the same time.